I have 10-15 years of experience and my base offer from MSFT (Bellevue) was $110k/year.

You were low-balled or have particularly shitty resume. 110k is offer for new grads. The right approach to maximizing offer is to get 5-6 offers, do bidding war between companies you do not want to work for and then ask the company you want to work for to match the highest offer.

They came out and told me they didn't think I had the skill level for the senior position I had applied to, but, they had a junior-level position open at that salary. It's more than I make now, but, I turned it down because it wasn't more enough to count for extra hours they expected, the higher cost of living (or commute), and the possibility of getting eaten by the performance evaluation system (at that time, was still the stack ranking and all the horrors it entailed).

Microsoft semi-(in)famously had a strategy of paying 70% of market salary.

When I was at Microsoft ~3 years ago, the strategy was to pay at the 70th percentile of market salaries, not pay 70% of the current market salary.

Sorry, you're right, 70th percentile. My point though is just that he may not have been lowballed deliberately.

That seems to jive with all I've heard with Google from some friends & acquaintences who are stellar in the Valley - is there anyone else at one of these big companies who can confirm/deny?

Ask a simple question, get a simple answer: yes.

The parent comment didn't say over 180k base, he/she said over 180k.

Google and Facebook (and some other companies) have upped the ante in compensation by offering lots of (liquid) stock.

It was several years ago, I'm assuming the market has only strengthened.